(1) Field of the Invention
The present invention relates to a heat-sensitive recording material. More particularly, the present invention relates to a heat-sensitive recording material having an excellent aptitude for a high speed recording, a high heat resistance, and a satisfactory whiteness.
(2) Description of the Related Art
Heat-sensitive recording materials in which a heat color-developing reaction of a colorless or light color leuco dye with an organic acid material, for example, a phenol compound or organic acid compound, is utilized are disclosed by, for example, Japanese Examined Patent Publication (Kokoku) Nos. 43-4160, 45-14039 and 48-27736, and are now widely used.
These heat-sensitive recording materials are advantageous in that colored images can be easily formed only by heating and the recording apparatus can be made relatively compact and small size, and thus are widely utilized as information-recording materials. Also, facsimile machines and printers for which the heat-sensitive recording materials are used have been greatly improved, and therefore, the formation of colored images at a very high speed, considered impossible or very difficult in the past, is now possible.
Due to the above-mentioned development of the high speed heat-sensitive recording machines, the heat-sensitive material to be used for the high speed recording machines must have an enhanced recording sensitivity, in comparison with conventional heat-sensitive recording materials, and many attempts have been made to meet the above-mentioned requirement. Most of these attempts relate to combinations of specific leuco dyes with the color developing agents, or to utilization of specific heat-fusible substances.
The heat-fusible substances are used as a sensitizing agent for the color developing reaction in the heat sensitive color developing layer and include, for example, the phenyl 1-hydroxy-2-naphthoate disclosed in Japanese Unexamined Patent Publication No. 57-191089, p-benzyl-biphenyl disclosed in Japanese Unexamined Patent Publication No. 60-82382, benzyl naphthyl ether disclosed in Japanese Unexamined Patent Publication No. 58-87094, dibenzyl terephthalate disclosed in Japanese Unexamined Patent Publication No. 58-98285, benzyl p-benzyloxybenzoate disclosed in Japanese Unexamined Patent Publication No. 57-201691, diphenyl carbonate and ditolyl carbonate disclosed in Japanese Unexamined Patent Publication No. 58-136489, m-terphenyl disclosed in Japanese Unexamined Patent Publication No. 57-89994, 1,2-bis(m-tolyloxy)ethane disclosed in Japanese Unexamined Patent Publication No. 60-56588, and 1,5-bis(p-methoxyphenoxy)-3-oxapentane disclosed in Japanese Unexamined Patent Publication No. 62-181183.
When a heat-sensitive color developing layer containing the heat-fusible sensitizing agent is heated, first the sensitizing agent is melted and the leuco dye and the color-developing agent is dissolved in the melt, whereby the molecule of the dissolved leuco dye and color-developing agent are mixed and reacted with each other to develop a color in the color developing layer.
Therefore, the heat-fusible sensitizing agent must have a proper melting point, preferably from 80.degree. C. to 110.degree. C., and be highly compatible with the leuco dye and the color developing agent.
Also, preferably the heat-fusible sensitizing agent does not cause a lowering of the whiteness of the heat sensitive color developing layer. Therefore, the heat-fusible sensitizing agent must be substantially insoluble in water and must not discolor the color developing layer.
When a certain type of conventional heat-fusible sensitizing agent is contained, a white powder-like substance appears on the resultant color developing layer, with a lapse of time; which is known as the color developing layer whitening phenomenon. The whitening phenomenon is believed to depend closely on the sublimating property of the heat-fusible sensitizing agent, and therefore, the heat-fusible sensitizing agent must have no or a very low sublimating property.
In a practical recording operation, frequently the heat sensitive recording materials are temporarily exposed to a high temperature of about 60.degree. C. to about 70.degree. C. Under such a condition, the heat-sensitive color-developing layer must exhibit a high heat stability and must not develop a color at that high temperature. Therefore, the heat fusible sensitizing agent must not affect the heat stability of the heat-sensitive color developing layer.
As stated above, many heat fusible sensitizing agents have been provided, but very few of them can meet all of the above-mentioned requirements, and thus a new type of heat fusible sensitizing agent is urgently required.